1. Field Of The Invention
The present invention relates to an apparatus for evaporating or "stripping" the volatile components of a polymer, such as polystyrene or polyester, and more particularly relates to a partial evaporator unit which is employed as a devolatilizer for a polymer and a method of devolatilizing the polymer using the apparatus of the present invention.
It is conventional to use a devolatilizer at the end of a polymer train, that is, the series of apparatuses used in the process of making a polymer such as polystyrene. The devolatilizer is used to remove the volatile or lighter components of the polymer.
The volatile or lighter components of the polymer may consist of unconverted monomers, which are of low molecular weight and low viscosity. These monomers may be the result of an incomplete conversion process, or they may constitute lighter components of the polymer which has broken down to form monomers, which monomers may be different from the monomers used to make the polymer. Alternatively, in some polymer processes, it may be necessary to add a diluent to adjust the viscosity of the polymer. This diluent must be eventually removed.
The volatile or lighter components of the polymer are removed by a partial evaporator or devolatilizer. The lighter components are removed by evaporation, which is commonly referred to in the art as "stripping".
To help "strip" the lighter components from the polymer, one tries to maximize the force driving the lighter components from the liquid to the gaseous phase in the evaporation process. One way of accomplishing this is by using a pull vacuum situated at the end of the polymer train.
Another method used in combination with the pull vacuum is to provide heat to the polymer to maintain its high temperature, because as vapor is released, the vapor takes heat with it from the polymer and the polymer cools down and becomes very viscous. A heat source is used and is disposed in thermal contact with the polymer to maintain its high temperature.
Also, it is desirable to increase the inter-surface contact area between the polymer and the heat source so as to increase the heat transfer rate or the amount of heat transferred from the heat source to the polymer.
It is also important to make it easy for the molecules of the lighter components to migrate to the surface of the polymer where they may change to a vapor phase. Therefore, one would make the polymer into a thin film, which is advantageous for mass transfer.